The disclosure relates to a flexible mounting assembly for an air frame structure that limits interaction between two structures on the air frame structure while providing a solid closeout between the two structures. As an example, a flexible mounting assembly as described herein may be used on an air frame structure surrounding an existing and self-supporting opening, for instance, a cargo and passenger bay area in a helicopter, thus limiting the interaction between top structural members of the helicopter on which engine components are mounted, bottom structural members defining the bottom of the cargo and passenger bay area of the helicopter, and vertical frame members extending between the top and bottom structural members of the helicopter. Limiting the interaction between the vertical frame members and the top and bottom structural members reduces stress loading at the connection points of these members.